<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .banner {
            width: 1226px;
            height: 600px;
            margin: 0 auto;
            position: relative;
        }

        .mask {
            height: 150px;
            width: 1226px;
            background-color: rgba(0, 0, 0, .5);
            position: absolute;
            bottom: 0;
        }
    </style>
</head>
<body>
<div class="banner">
    <img src="images/bg.jpg" alt="">
    <div class="mask">

    </div>
</div>
</body>
</html>